The role of the renin-angiotensin-aldosterone system in the control of blood pressure and in the maintenance of electrolyte and acid-base homeostasis is being investigated in normal subjects, patients with hypertension of renal or adrenal origin, patients with chronic renal insufficiency and in patients with primary abnormalities in renal tubular transport. Studies of circadian variation of aldosterone, cortisol, deoxycorticosterone, corticosterone and angiotensin levels may indicate the factors responsible for the adrenal secretory patterns that characterize certain renal and adrenal disorders. Metabolic balance studies can provide quantitative data with respect to the effect of hormonal factors in renal potassium and acid-base metabolism. Administration of agents that inhibit angiotensin I converting enzyme activity (SQ 20881 and SQ 14225) should provide further insight into the role of the renin-angiotensin system in the genesis and maintenance of a hypertensive state and may also represent a major advance in the treatment of hypertension. Continued efforts to develop a rat kidney receptor assay may prove useful in the detection of patients with hypertension due to as yet unrecognized mineralocorticoid hormones.